In media handling assemblies, particularly in printing systems, accurate and reliable registration of the substrate media as it is transferred in a process direction is desirable. In particular, accurate registration of the substrate media, such as a sheet of paper, as it is delivered at a target time to an image transfer zone will improve the overall printing process. The substrate media is generally conveyed within the system in a process direction. However, often the substrate media can shift in a cross-process direction that is lateral to the process direction or even acquire an angular orientation, referred herein as “skew,” such that its opposed linear edges are no longer parallel to the process direction. Thus, there are three degrees of freedom in which the substrate media can move, which need to be controlled in order to achieve accurate delivery thereof. A slight lateral misalignment, skew or error in the arrival time of the substrate media through a critical processing phase can lead to errors, such as image and/or color registration errors. Also, as the substrate media is transferred between sections of the media handling assembly, the amount of positioning error can increase or accumulate.
Contemporary media handling systems attempt to achieve position registration of sheets by separately varying the speeds of spaced apart drive wheels to correct for skew and/or lateral mispositioning of the sheet. Such systems that separately vary the drive wheel speeds are commonly referred to as differential drive systems. The drive wheels are used with cooperating idler rollers for engaging the substrate media there between. The differential drive wheels with the idler rollers are together referred to as differential nip assemblies.
Examples of typical sheet registration and deskewing systems are disclosed in U.S. Pat. Nos. 5,094,442, 6,533,268, 6,575,458 and 7,422,211, commonly assigned to the assignee of record herein, namely Xerox Corporation, the disclosures of which are each incorporated herein by reference. While these systems particularly relate to printing systems, similar paper handling techniques apply to other media handling assemblies. Such contemporary systems transport a sheet and deliver it at a target time to a target location, based on measurements from the sheet sensors. The target location can be a particular point in a transfer zone, a hand-off point to a downstream nip assembly or any other target location within the media handling assembly. Typically, based on sheet sensor measurements, a controller can adjust the sheet velocity to steer the sheet to a target location at a desired time. The controller uses the differential drive system to correct primarily for skewed positional errors detected for the sheet. Temporarily driving two motors at slightly different rotational speeds induces a rotational sheet motion that is used to eliminate/correct for detected skew and/or process timing errors. The resultant dynamics are nonlinear and make closed-loop feedback control complex and difficult to execute.
Other contemporary systems use alternative cross-process correction techniques, such as nip assemblies that translate laterally in order to shift the sheet while engaged within the nips. However, laterally translating nip assemblies include driven wheels mounted on a moveable carriage assembly. Driven wheels inherently include motors, gears and/or belts associated therewith, thus such assemblies are complex, costly, prone to mechanical failure and difficult to repair. Also, having to reset the mechanical carriage between sheets limits the speeds and inter-copy gaps at which the system can function.
Another alternative system uses nip assemblies with fixed angled driven wheels that drive the sheets into a straight edge fence or rail, thereby correcting both cross-process and skew errors simultaneously. However, such systems are limited in the size and type of substrate media being handled and are prone to marking, buckling or damaging the substrate media.
Accordingly, it would be desirable to provide an apparatus for and a method of registering the lateral position of a sheet in a media handling assembly, which overcomes the shortcoming of the prior art.